Lasers have been used to pump, that is, inject laser light into optical fibers, such as amplifier fibers. High power fiber lasers and amplifier fibers are being developed for remote sensing, target identification, high power directed energy applications, and long-distance free-space optical communications. Novel fiber designs involving double clad fibers with enlarged core sizes and mixtures of rare earth ions have facilitated optical systems with over 100 W of optical power. The pumping of such systems is a key step for reaching still higher output power levels. Most semiconductor lasers will emit light normal to the exit facet at the end of the laser waveguide. When the laser light from the facet is directed from any side into the fiber, the laser light will not be guided through the fiber but will exit the fiber after a single crossing of the fiber. Hence, a redirection of the laser light along the fiber is necessary. Current systems use various methods for redirecting and coupling the pumped laser light into optical fibers.
End pumping methods include end optical pumping using a dichroic mirror or using a concentric set of fibers that are fused and tapered together to connect several delivery fibers to the end of an amplifier fiber. End pumping is convenient to implement. However, end pumping becomes more difficult as the power level is increased due to crowding of the pumps, which need access to the end fiber facets. Additionally, there is limited control over the optical excitation profile along the length of the fiber amplifier because light can only be launched from two points, the front or the back whereas side-pumping allows for tailoring of the excitation profile as desired. End pumping is restricted to only laser pumping from the ends of the fiber.
Groove methods provide for cutting grooves into the glass, and launching light from the side of the fiber onto these grooves for reflecting the laser light into the inner cladding. Using grooves for side coupling severely debilitates the mechanical strength of the fiber. Forming reflecting grooves is a tedious and difficult process, particularly when the intrinsic performance of the fiber is to be unaffected by the grooves. Furthermore, under certain conditions, the grooves can couple a significant amount of light out of the fiber, when not designed and placed properly.
Directional side-pumping methods remove part of the secondary polymeric fiber cladding and provide pump light by a contacting pump fiber. Directional side-pumping methods are currently being used in commercial products. The directional side-pumping method is effective, but is not conducive for coupling high-power laser diode arrays. The directional side-pumping method is essentially limited to single element devices and undesirably requires that a long contact length be maintained between the pump fiber and the amplifier fiber. The directional side-pumping method disadvantageously requires intermediate fibers or optics, such as lenses, between the laser diode pump source and the pump fiber. These intermediate free space optics complicate system designs.
High power optical amplifiers and fiber lasers make use of amplifier fibers. An amplifier fiber includes a polymeric cladding, a hexagonal inner cladding for confining pump laser light, and an inner most signal light core that communicates the laser light to be amplified. For example, the Er or Er/Yb doped amplifier fibers are components for high bit rate optical networks. Reliable and high power laser diodes are the essential devices for pumping these amplifying fibers. These laser and fiber systems require high fiber coupling efficiency. High coupling efficiency usually requires that the lasers have high spatial beam quality. Broad area multimode lasers have inherently broad area beams. Tapered lasers provide a solution in terms of high brightness with high power and high spatial beam quality. High bit rate optical networks using these doped fiber amplifiers may use reliable high power laser diodes operating at 980 nm for pumping these fiber amplifiers. Current methods do rely on improved spatial beam quality of high power laser diodes for efficiently pumping the optical fiber. These and other restrictions are solved or reduced using the invention.